Method and apparatus for neutron well logging



July 18, 1950 J. M. THAYER Er AL METHOD AND APPARATUS FOR NEUTRON WELLLOGGING Filed July l0, 1948 3 Sheets-Sheet l July 18, 1950 J. M. THAYERET Al. 2,515,535

METHOD AND APPARATUS FOR NEUTRON WELL LOGGING Filed July lO, 1948 5Sheets-Sheet 2 WMNWNM July 18, 1950 J. M. THAYER Er AL.

' METHOD AND APPARATUS FOR NEUTRON WELL LOGGING Filed July l0, 1948 5Sheets-Shes?l 3 Swim/wromV .Mw/MQ www Patented July 18, .1950

'METHGDANDAPPARATUS FOR NEUTRON WELL LOGGING" Jean Thayer,.Gilbert..Swift, anclRobert E.,

Fearon, ,T.ulsa, Okla., assignors to Well Surveys, Incorporated,.Tulsa,Okla., a corporation of Delaware Application July 10, 19,48; Serial No.38,138.

(Cl. 'Z50-83.6,)

12 Claims.

This inventionfrelatestothe artlof -subsurface* exploration,.principallyoil well-logging, and more particularly ,toa radioactivity typeofexploration in which-,a sourceoffast neutrons is used in conjunction-with a gamma-radiation detector. Cemmercially-,sucha radioactivity logmade by the use of av source ofufastneutrons anda gammaradiationdetector is known as a neutron log. This is truedespite the fact'that noneutrons are directly detected.

vInrecent yearszneutrono'oil well logs vhave achieved AVa :degree ofpopularity 4not shared by theV logs made -by other-methods. This isbelieved to be attributable to the fact that,in asubstantial-proportionofsurveys made, they correlate-more.accuratelyvvith thelithology of the strata penetrated by the well.lThese logs have been made by traversing the.well;with a source ofrneutrons, usually 300-to 5001mi11icuries of radium intimately rmixedwith a predominant proportionrby Weight of powdered beryllium, toirradiate with fast vneutrons `the strata lining the well andsimultanecusly'traverse the well with anassociatedgamma-radiation,detector to detect and record gammaradiation incorrelation with the depth. at which theyvare detected. The detector,for: example,` an :ionization chamber, and the source are iassembledtomake a single unit,` withythe: detector `vertical-1y spacedY from thesource.

We have discovered that,;regardless ofthe type of neutron source used`in neutron Well logging, there-is recorded Ialong with they desiredeffect, namely, the intensity of Ygamma/radiation originatingwithneutron processes inthe'formations,

an undesiredeifectWhich should be minimized orf-largely eliminated4 from#the neutron log. This effect occurs at randoni1 intervals of time andis `evidenced 4byV sudden transitions, -or fluctuations, of appreciablemagnitude .in the trace of thelog; Whenxthese transitions Loccur on thetrace, along 'with transitionsof comparable magnitude.whichareoccasioned vby.L changes in the lithological characteristics ofthe formations, the log is incapable ofxbeing properly interpreted.Furthermore; these `random transitions, depending on the timelofcoccurrence 4and the direction ofthe transition: on :the .logxthat is`due to a change in the-lithological-characteristics of the formations;can cveremphasize,y obscure, .or even lreverse the wanted. transition.Infact, the degreeofreproducibility cfa log-fis measured by therelative-magnitudes.1ofathe `unwanted random transitions andthe 'wantedtransitions that are occasioned bin-variationsinrlithologicalcharaclteristicsrofthe:formations We. have discoveredthatY these random fluctuations or transitions are attributable .to twoeffects. One of these eects is that which is inherent in all radiationemitters, namely, the statistical variation in theuintensity of .theneutrons emitted. by the. source. The other effect is that produced -,by,neutron-heavy ionizing-particle processes ,occurring within thegamma-ray detector.

The rst of .thesefeffects .can be vminimized or vlargely eliminated byusing a neutronsource that is `of such strength that the ratio vofdesired effect to .undesired effectis sufficiently great that thesensitivity of the recording system can bereducedto a point. where theundesired. effect is largely eliminated. and the desired .effect canstill berecorded `without .losing the. characteristics which serve as..anindex to the formations being surveyed.

The second effect Vis attributable to neutrons which .have passed .moreor less directly from `the source. to the vinteriorcf the...detector andthere reacted withsomematerial inside the. detector (in .an ionizationchamber, for example,

the aluminum of which the central electrode .is

formed, theironcnsteel of which the outer electrede is fo1med,.'or theionizable medium) to producea proton .or analpha particle. .Protons .oralpha particles, in theirpath of `travel through theionizable medium Vinthe detector, produce enough ions, to ca-use a considerable variation inthe current output from the detector. Fora detector having vagiven:cross-sectionalarea, the cp-portunitiesfor producing this effectvary with the distance `between the` source-and the detector inaccordance with the. inversefsquare law, multiplied by the .absorptioneffect. for .intervening material. rlhe randomness of theeffect,however, is attributable to the tact` that only occasionalneutrons arecaptured andrelease energy the producition of protons or alpha particlesin the detector. .We havefound that the average ,rate ofoccurrenceofthese processes can be reduced by increasing the distancebetween the detector and the neutron source, and by reducing .thecross-sectional area of the detector to ynresent a smaller target forthe direct neutrons.

,Neither of these dimensional factors, however,

and the detected gamma radiation arisingfrom neutron processes in theformations will increase, resulting in a more intense component ofuseful current flowing in the detector circuit. While the increase inthe number of neutrons emitted per unit of time by the source willproportionally increase the opportunities for neutron-proton orneutron-alpha particle reactions to occur, we have nevertheless foundthat the resultant effect is only an increase in the average rate ofoccurrence of these reactions and not an increase in the magnitude ofthe ionizing process produced by each particle released in the ionizablemedium. Therefore, the use of a stronger source increases the intensityof the wanted component of current flowing in the detector circuitwithout correspondingly increasing the magnitude of the fluctuations dueto the random processes. The sensitivity of the detecting system canthen be reduced to minimize its response to the random processes withoutseriously impairing the useful intelligence depicted by the log.

Although certain ways have been described above for minimizing theeffect produced by these random neutron processes, we have alsodiscovered that this effect can be largely eliminated. This can beaccomplished by using a detector which employs as an ionizable medium asubstance which does not emit heavy ionizing particles when bombardedwith neutrons, and by forming all metallic surfaces that are exposed tothe ionizable medium inside the detector of an electrical conductor thatwill not emit heavy ionizing particles, such as protons and alphaparticles, when bombarded with neutrons. We have discovered that argonand tin, respectively, are ideally suited for these purposes. Theelectrically conductive elements of the ionization chamber which havesurfaces exposed to the ionizable medium inside the chamber need not beformed wholly of tin but may be plated with tin to a thickness, forexample, of a few thousandths of an inch, which plating will not emitheavy ionizing particles when bombarded with neutrons, and will absorbheavy particles, such as protons and alpha particles, that are emittedby the underlying plated metals when they are bombarded with neutrons,thereby preventing the heavy particles from reaching the lionizablemedium in the chamber. Tellurium may be similarly employed.

It is the primary object of this invention to determine the causes ofcertain sudden random fluctuations which have occurred in and impairedneutron logs heretofore made, and to provide a solution for thisdifficulty. It is an object of the invention to minimize the effect ofthese random fluctuations on the log to tolerable proportions byincreasing the desired radiation reaching the detector withoutcorrespondingly increasing the magnitude of said fluctuations. It isalso a object to reduce the frequency of occurrence of these randomfluctuations on the log by minimizing the cross-sectional area of thedetector and by enlarging the spacing between the detector and theneutron source without, however, seriously disturbing the optimumspacing requirements based upon other considerations. It is a furtherobject to minimize, or wholly or largely eliminate, these randomfluctuations by ascertaining and suppressing the phenomena from whichthey result. It is a speciiic object of the invention to accomplish thisby forming surfaces that are exposed to the ionizable medium inside thedetector of a substance that will not emit heavy ionizing particles whenbombarded by neutrons, and that will absorb such particles that may begenerated by bombardment of nearby metal parts and that otherwise mightfind their way into the ionizable medium. It is a further specificobject of the invention to minimize the opportunities for this effect tooccur by using in the detector a substance as the ionizable medium whichwill not emit heavy ionizing particles when bombarded with neutrons.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when considered withthe drawings, in which Figure 1' is a diagrammatic illustration of aneutron logging operation;

Figure 2 is an enlarged vertical sectional View of the subsurfaceinstrument;

Figures 3a, 'and 3b are standard neutron logs which have beensuccessively made in the same well which illustrate the effect of randomtransitions on thev reproducibility of neutron logs;

Figure 3c is a record of background noises and random transitions madeby stopping the subsurface instrument in a well and allowing therecorder strip to be driven at its normal speed;

Figures 4a to 4c illustrate the manner in which the number of neutronsentering the detector varies with the spacing between the neutron sourceand the detector and also the paths followed by the useful radiation inthe formations; and

Figure 5 lis a fragmentary view of the subsurface instrument showing invertical section details of an ionization chamber type of gammaradiationdetector.

Referring to the drawings in detail, particularly Figure 1, there isillustrated a well surveying operation in which a fragment of thesurface I0 of the earth is shown in vertical section. A well IIpenetrates the earths surface and may or may not be cased. Disposedwithin the Well is the subsurface instrument I2 of the well loggingsystem Which additionally comprises a cable I3 for suspending theinstrument in the well, a drum I4 from which cable is payed out or onwhich cable is wound when causing the capsule I2 to traverse the well,electrical connections from sliprings on the aXle of the drum I4 to anamplier I5, which in turn is electrically connected to a recorder I6 ina conventional manner. Recorder I6 is driven through a transmission I'Iby the drum I4 as the cable is payed out from or wound thereon. Therecord thus made by the recorder as the capsule I2 traverses the drillhole will be in correlation with depth.V

As shown in Figure 2, the capsule I2 comprises a neutron source I8forming the bottom portion thereof and a gamma-radiation detectingsystem indicated generally as I9 which makes up the upper portion of thecapsule. The gammaradiation detecting equipment I9 can be such as thatdisclosed in Patent No. 2,349,225 or such as that shown in Patent No.2,390,965. For purposes of describing this invention an ionizationchamber 20 is shown as the radiation sensitive element. As disclosed inPatent No. 2,308,361, the operation of a system of this character whenproducing a neutron log is that the capsule I2, made up of a source ofneutrons I8 and a gammaradiation detection system I9, is caused totraverse a well. Neutrons emitted from the source enter the walls of thewell and, by nuclear reaction with matter contained in the walls, pro- 5duce gamma radiation in amounts proportional to theslithologicalcharacteristics of the.4 materials of which the Walls are formed. Thesegamma radiations produced'by .nuclear reactions in the strata aredetected by the gamma-radiation detector 2t by producing electrica-lsignais that are related in magnitude` to the intensity of thegammaradiation detected, andthese-signals are amplified by anamplierZiandv transmitted over conductors contained in the.cable i3 to thesurface of the earth,.where if necessary, they are further amplied bytheamplifierY I5 'and recorded by the recorder I6 in correlation with thedepth at which they weredetected. It isv to be understood that thepresent-inventionisinot limited to an ionization chamber-type .detectorbut applies to counters as Well.

commercially a; log made by the above-described operation is known as aneutron log.

This is true although-no neutrons are-directlyv detected and recorded.The record, is oneof gamma radiation intensity versus depth. Thoseworking in the arthave heretofore assumed that such a log trulyrepresents an effect produced in the strata by irradiatlng thestratawith neutrons. That is, the log was purported to bea measurementof the gamma-.radiation produced Y by thenuclear reaction of `nelrtronsand elements contained in the strata; versus depth. Research,

however, has shown us `that inV many instances the standard neutron` logis'unreliable since it cannot be reproduced in the same .well while'working under thesame conditions.` We have discovered that theinability lWto.. reproduce the standard neutron log under the sameworking conditions is attributable to the presence on the log of randomtransitions that are of substan-I tially the same order of magnitude asthe transitions that arecaused by changes in lithologicalcharacteristics of the formations being surveyed.

The manner in which these random transitions affect the neutron log isillustrated in Figures 3a to 3c.` Figures 3a and 3b are two neutron logswhich were made in the same well under identical conditions. Figure 3cshows a record which was made by stopping the subsurface instrument,k

at a particular depth in the well and driving the recorder strip at itscustomary speed. The trace, so recorded, is a record ofthe randomtransitions that occur versus time. Nowy compare the two logs of Figures3a and 3b While keeping in mind the magnitude of the random transitionsshown on the trace of Figure 3c. 1 At a glance the logs appear to beduplicates. but a careful examination of them will show that they differat many points, particularly where the transitions are small. Thetransitions at K, L, M, N, O, and P, for example, are unquestionably dueto changes in lithological characteristics of the formations beingtraversed. Thesel transitions, although small in magnitude, are in phaseand are of approximately the same magnitude. logs of the same wellbeforehim, the interpreter can say with certainty that these aretransitions caused by change in for-mations, because they appear atexactly the same point on both-logs. Transitions. such as those shown onthe logoi Figure 3b at Q, R, S, T, and U, may, or may not, be due tochanges of lithological characteristics of the formations. They are ofthe same order of magnitude as the transitions at K, L, M, N, O, and P,but they do not appear on both logs. Since the transitions at Q, R, S,T, and U` are also of the same order of magnitude as the randomtransitions shown Aonthe trace in FgurefSc,

With two villustrated in Figures 4a to4c.

itv is possible that lthey `are random transitions, such as those Yonthe trace of Figure 3c. yThese transitions therefore become meaninglesseven when two logs of the samey well are considered. An interpretationof the small transitions of either one of the logs. whenconsidered-alone, would be impossible: for the interpreter would notknow'which of the small transitions are attributable to changes informations and which are Ydue-to. random processes. Additionally,important formation changes would not be-portrayed by the log if an -outof phase random processoccurred at the same time that the formationbeing surveyed changed. Therefore, depending upon the direction of thetransition due to formation change, a random transition can over orunder accentuate a desired transition, or it can even nullify thedesired transition.

AAs .pointed out above, these random transitions are attributable totwo-*.causes, one of which is the statistical variationA in the`intensity ofthe neutron flux emitted by the.v source and the other isneutron-heavy4 lionizing particle processes which occur in the detector.

We have discovered that the random transitions that arey due-tostatistical variations inthe neutron ilux emitted by the source can besufficiently minimized by increasing the neutronux emitted vby the-source to approximately 5.x 106 neutrons -per second. This can` beaccomplished by increasingtheamount of alpha rayer in the mixture withberyllium. Increasingthe source strength results in an increasev in theintensity ofthe gamma radiation produced` in the formationsV anddetected by the measuring instrument. A direct consequence of this is anincreased ratio of magnitudeof useful signals to the magnitude ofthesignals representing. statistical Variations. A ratior of v-at `least 3to 1 will make logs which can-be reproduced Awith the desired degree ofcertainty. Thisy allows the sensitivity.V of the recording system to bereduced to a point where the random transitions/are. minimizedY orlargely eliminated from the trace of the log andthe transitions due tochanges` in formation are still recorded without appreciable loss ofcharacter.

Nowv consider the ran-dom transitionsthat are recorded on thetrace ofthe neutron log and which are caused by neutron-heavy particle processesthat occur in the'gamma-radiationdetector. These transitions arecausedby neutrons travelling more orlessfdirectly` from the source intothedetector and there reacting withsome substance, such las: thesubstance of which the electrodes are ma'deior'witlr-the ionizablemedium itself, toproduce protons or alpha particles. The protons. oralpha particlesso produced wil1,.in theirpaths of travel throughftheionizable medium, produce numerous electrons which arecollected bythedetector-collectorV electrode, resulting inV bursts of` current flowinthe electrode circuit w-hich, when amplified Aand recorded, produceoutstanding randomY transitions. These random transitions ycan -a-lso beminimized'r in the manner describedabovefby increasing the strength 0fthesource.

We have also discovered that the random transitions that areIattributable to neutronheavy particle reactions that occurinthedetector can also beA minimized by criticallyspacing theneutronsource fromk the detector. This is Fast neutrons are emitted from thesource inf all directions and the number which enter a= detector havinggiven dimensions,- and which-have an opportunity to 7 react with asubstance therein, varies inversely as the square of the distancebetween the point where the neutron-producing reactant materials arelocated and the detector. In Figures 4a to 4c there are shown threeconditions of spacing of neutron source from the detector in a Well.Figure 4a shows generally the paths lc of the neutrons which enter thestrata and there produce gamma radiation. The useful gamma radiation soproduced reaches the detecting instrument by the paths Z. Although thepaths 7c and Z are shown only on one side of the instrument, it is to beunderstood that the neutrons are emitted in all directions and the pathsappear on all sides of the instrument. Neutrons which travel directlyfrom the source to the detector follow the paths m. The number ofneutrons that follow the paths m and enter the detector vary inverselywith the square of the distance between the source and detector. Aspointed out above, not all of the neutrons which enter the detectorproduce protons or alpha particles which contribute, by their ionizationprocesses, a coinponent of current to that ilowing in the electrodecircuit.

For the purpose of comparing the eifects producedV in the detector bythe gamma radiation produced by neutrons in the strata and the protonsor alpha particles produced in the detector by neutrons which havetravelled substantially directly from the neutron source, let us rst,for

purpose of explanation only, assume that -one out of each of n neutronsentering the detector produces an ionizing proton or alpha particle, andassign arbitrary values to the current components that would flow in theelectrode circuit. For the spacing shown in Figure 4a, let us assign avalue of 3 to the component of electrode current due to gamma raysproduced in the strata byneutrons, and .O3 to the component of currentthat would flow in the electrode circuit due to ionizing processesproduced by protons. We would than have a ratio between the componentsof 100 to 1. Now referring to Figure Liin-the spacing between the sourceand detector shown there has been decreased to a point where thatcomponent of current that would flow in the electrode circuit due togamma rays produced in the strata by neutrons would be doubled. Thisreduction of the spacing would, under the assumption made in connectionwith Figure 4a, now increase the current component due to protons to.12, giving a magnitude ratio of useful signal to undesired signal of 50to 1, instead of i the 100 to 1 obtained under the condition of Figure4a. In Figure 4c the source is shown still closer to the detector,spaced therefrom by a distance that will cause the useful signal toincrease to 12. That component that would ow in the electrode circuit,the unwanted signal, would increase to approximately .5,reducing theratio of desired signal to unwanted signal to 24 to 1. It is seen thatthis undesired effect increases from that for long spacing betweensource and detector slowly at rst as the spacing decreases and rapidlyfor very short or close spacing. Use of a relatively great spacing thusserves to minimize or even largely eliminate this phenomenon. Too greata spacing, however, results in the decrease of the wanted signal to suchan extent that the repeatability of the log is impaired. Thus it is seenthat as the spacing between source and detector is varied from veryclose to very far the repeatability at rstimproves, as the processesinthe detector caused directly by neutrons diminish and later againbecomes poor as the intensity of the wanted radiation diminishes. Thereare therefore maximum and minimum limits of spacing which can beemployed with a source of given strength in order to achieve anacceptably repeatable log at a reasonable logging speed. When usingsources of scarce or costly materials these limits become of greatimportance in order that satisfactorily repeatable logs may be obtainedwith a minimum amount of source material. We have found that when usingdetectors in which no attempt has been made to minimize or eliminate thedirect interaction of neutrons with the materials inside the detectorthat the satisfactory limits for operation of a weak'source, such as onewhich emits l06 neutrons per second, lie between 6 and 14 inches. As thestrength of the source is increased, as for example to 'one which emits0.5 107 neutrons per second, the satisfactory range of spacingsincreases to 4 to 20 inches. As the eiects inside the detector producedby direct interaction with neutrons are-reduced theminimumsatisfactory'- spacing decreases. For example, with a detector inwhich this effect has been reduced by a factor of ten from that of adetector of the type shown in Figure 5 and more fully disclosed inPatent No. 2,390,965, the minimum permissible spacing fora weak sourcewill decrease from l6 to about 3 inches, and with a detector in whichthis effect has been entirely eliminated a spacing of zero inches,meaning that the neutron source is located within the detector, can betolerated.

We have further found that the random transitions that are attributableto neutron-heavy particle reactions inthe detector can be largelyeliminated by forming all metallic surfaces that are exposed to theionizable medium inside the detector of a metal that will not emit heavyionizing particles, such as protons and alpha particles, when bombardedby fast neutrons, and preferably employing an ionizable medium whichalso will not emit heavy ionizing particles when bombarded by fastneutrons. One such detector is shown in vertical ysection in Figure 5.Although this detector forms a part of the subsurface system that iscontained in a capsule, only that fragmentof the capsule which housesthe detector is shown.

Referring to the drawings, the capsule or casing 22 is divided into aplurality of compartments, one of which, compartment 23, contains anionization chamber that is defined by the inner walls of the casing 22and top and bottom partitions 24 and 25, respectively. The ionizationchamberv thus formed contains an ionizable medium such as argon for thedetection of gamma radiation. There are concentrically disposed in theionizable medium within the ionization chamber two electrodes, an outercylindrical electrode 26 and a central electrode 21. The outer electrodeis i'ixed in spaced relation to the casing 22 by means of a dielectricmaterial 28. Since the ionizable medium in the chamber is under that iscarriedby the outer shelly and the elongation'. ofnthe inner endof-:thetcentral electro'defof` the plug'. i i

-The'ibottom end-of 1the-ionization chamber centralr electrode is`supported by an insulator 3 l.` Theinsulator is secured to a tubularelement 32 that' is adapte-:l totelescopically` engage-the innersurfaceof i thef tubular'A central electrode 2l. Element 32 is'adaptedtolitsnuglyinside the Y bythe spring 33 Ito engageafbeari-ng cupy35 thatis formed in an upraised` portion ofV the center ofpa'rti-tion25. Passag-ewaysll` are formed horizontally" intheupraised portion ofpartition andthese passagewayscommunicate with a central opening t? inwhich is secured a valve 313-..`

Valve S8 is 4provided for the purpose of charging the ionization`chamber-with an ionizable medium, suchmas argonf Such an ionizationchamber is more fully disclosed in Patent No. 2,390,965.

The novel features of the present invention as applied to such a chambercomprise using an ionizable medium such as argon and formingallelectrically conducting surfaces inside the chamber that are exposedtothe ionizable medium with an electrical conductor, such as tin oritellurium, which `mediumand electrical conductor will not emit 4heavyionizing particles, such as protons or alpha* particles, when bombardedwith fast neutrons- -`This ca-n -bedone by making .the electrodes 2S and21 within the ionization chamber of tin or tellurium, or by coating orplating them with tin or tellurium to a thickness of at least 0.002inch. The inner surface of Ahousing 22 which is exposed to the ionizablemedium as Well as the inner surfaces of partitions 24 and 25 should alsobe coated or plated with tin or tellurium. When using a coating orplating of tin or tellurium, even though neutrons pass through thecoating or plating and react with the plated metals, all heavy particlesproduced thereby would be absorbed by the tin or tellurium and thus notbe allowed to enter the ionizable medium to produce the undesiredeffect.

In the construction of such devices as are herein described it isnecessary to be careful to eliminate boron and lithium from the internalelements of the gamma-ray detecting device.

The elimination, as just described, of the undesired effect produced byneutron-proton or neutron-alpha particle reactions in the detector makesit possible to reduce the spacing between the neutron source anddetector when desired to augment the desired effect from the strata ofrock.

The claims of this application are directed to a method and apparatusfor minimizing or largely eliminating random transitions, such as thosecaused by fast neutron-heavy particle processes or slow neutron-boron or-lithium processes, which occur in the detector. Other novel featuresdisclosed but not claimed are being made the subject matter of companionapplications.

We claim:

1. In a neutron logging detector, containing an ionizable medium, theimprovement which consists in surfaces exposed to said ionizable mediumconstituted of a material which does not emit heavy ionizing particlesWhen bombarded by neutrons.

2. A detector for neutron logging, comprising anlionizable gas, in whichdetectorthesurfaces of all electrically conducting elementsexposed tothe ionizable gas within the detector are composed ofsmaterial that hasthe property of abesorbing protons .andalpha particles and that does notemitheavy ionizing particles when bombarded With neutrons.

3i. Apparatus'for neutron well logging Which comprises a neutron source,a gamma-radiation zdetector, said gamma-radiation detector containinganionizable medium and having all surfaces .exposeol to `said ionizablemedium constituted of a material which does not emit heavy ionizingparticles when bombarded by neutrons,

:means for traversing thev well with said neutron source and'detector,means for recording detected gamma radiation. against depth in the form`of a log, and means for maintaining said source and saidf'd'etector in:xed relative spaced relationship .alongthe axis of the well. f

4t Apparatus for neutron well logging which comprises a neutronsource',a Vgamma-radiation detector; said gamma-radiation detector containing!van ionizable. gas and having! the surfaces fof all electricallyconducting elements exposed to thet ionizable gas Vwithin thedetectorlcomposed of material that has the property of absorbing protonsand. alpha' particles and that does not emit heavy ionizing particleswhen bombarded lwithneutrona means for traversing the well with saidneutron source 'and detector, means for'recording detected gammaradiationagainst depth inthe formof a well;log,'and means formaintaining said fsource and said detector in Jxed" relative spacedrelationship along the axis of the well.

5. Apparatus for neutron Well logging which comprises a neutron source,a gamma-radiation detector, said gamma-radiation detector containing anionizable gas and having the surfaces of all electrically conductingelements exposed to the ionizable gas within the detector coated with amaterial that has the property of absorbing protons and alpha particlesand that does not emit heavy ionizing particles when bombarded withneutrons, means for traversing the well with said neutron source anddetector, means for recording detected gamma radiation against depth inthe form of a well log, and means for maintaining said source and saiddetector in xed relative spaced relationship along the axis of the gWell.

6. Apparatus for neutron Well logging which comprises a neutron source,a gamma-radiation detector, said gamma-radiation detector containing anionizable gas and having the inner Wall of the detector housing and theelectrodes of the detector composed of material that has the property ofabsorbing protons and alpha particles and that does not emit heavyionizing particles when bombarded With neutrons, means for traversingthe well with said neutron source and detector, means for recordingdetected gamma radiation against depth in the form of a Well log, andmeans for maintaining said source and said detector in xed relativespaced relationship along the axis of the well.

7. A detector for neutron logging, comprising an inoizable gas, in whichdetector the surfaces of all electrically conducting elements exposed tothe ionizable gas Within the detector are composed of tin.

8. A detector for neutron logging, comprising an ionizable gas, in whichdetector the surfaces of all electrically conducting elements exposed tothe ionizable gas within the detector are composed of tellurium.

9. Apparatus for neutron Well logging which comprises a neutron source,a gamma-radiation detector, said gamma-radiation detector containing anionizable gas and having the vsurface of all electrically conductingelements exposed to the ionizable gas within the detector composed oftin, means for traversing the well with said neutron source anddetector, means for recording detected gamma radiation against depth inthe form of a Well log, and means for maintaining said source and saiddetector in fixed relative spaced relationship along the axis of thewell.

10IApparatus for neutron Well logging which comprises a neutron source,a gamma-radiation detector, said gamma-radiation detector containing anionizable gas and having the surfaces of all electrically conductingelements exposed to the ionizable gas within the detector composed oftellurium, means for traversing the well with said neutron source anddetector, means for recording detected gamma radiation against depth inthe form of a well log, and means for maintaining said source and saiddetector in xed relative spaced relationship along the axis of the Well.

11. Apparatus for neutron well logging which comprises a neutron source,a gamma-radiation detector, said gamma radiation detector containing anionizable gas and have the surfaces of all electrically conductingelements exposed to the ionizable gas within the detector coated withtin, said coating having a thickness of at least .002

inch, means for traversing the wellwith said neutron source anddetector, means for recording detected gamma radiation against depth inthe form of a Well log, and means for maintaining said source and saiddetector in xed relative spaced relationship along the axis of the well.

12. Apparatus for neutron well logging which comprises a neutron source,a gamma-radiation detector, said gamma radiation detector containing anionizable gas and having the surfaces of all electrically conductingelements exposed to the ionizable gas within the detector coated withtellurium, said coating having a thickness of at least .002 inch, meansfor traversing the well with said neutron source and detector, means forrecording detected gamma radiation against depth in the form of a welllog, and means for maintaining said source and said detector in xedrelative spaced relationship along the axis of the well.

JEAN M. THAYER.

GILBERT SWIFT.

ROBERT E. FEARON.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,303,688 Fearon Dec. 1, 19422,349,712 Fearon May 23, 1944 2,469,462 Russell May 10, 1949 2,469,463Russell May 10, 1949

